From the past, various speed control devices for maintaining vehicle speed at fixed levels have been known. According to such a speed control device which is sometimes called as a cruise control device, the driver is not required to keep stepping on the accelerator pedal to keep the automobile cruising at a constant speed and he is free from the need for adjusting the depression of the accelerator pedal in order to maintain a constant speed irrespective of the inclination and other conditions of the road.
Vacuum actuators which derive power from negative pressure of the engine intake system are commonly used as actuators for vehicle speed control. A conventional typical vacuum actuator comprises a diaphragm which defines a negative pressure chamber in cooperation with the casing of the actuator and a plurality of solenoid valves which selectively communicate the negative pressure chamber with the intake system of the engine or the atmosphere as required, and the resulting displacement of the diaphragm is transmitted to the accelerator pedal by way of a control cable. The solenoid valves are controlled by a control device incorporating a micro processor, and the output of a speed sensor is supplied to the control device. Thus, using the vehicle speed as a controlled variable and the accelerator pedal depression as a manipulated variable, the control device controls the accelerator pedal depression by way of the solenoid valves and maintains the vehicle speed at a constant level by a feedback control.
Specifically, negative pressure from the engine intake system is supplied to the negative pressure chamber by wa of a negative pressure valve when the accelerator pedal depression is required to be increased, and the atmospheric pressure is introduced into the negative pressure chamber by way of a vent valve when the accelerator pedal depression is required to be reduced. Additionally, when the accelerator pedal is required to be quickly released, for instance when the vehicle brake is activate, a safety valve is activated and quickly communicates the negative pressure chamber with the atmosphere. Thus, in order to assure a high level of reliability, the vent valve and the safety valve are used in parallel in a redundant manner. Japanese Patent Laid-Open Publication No. 62-96144 (based on US patent applicatoin No. 783,039 filed on Sept. 30, 1985) discloses a vacuum actuator of this type.
In assembling this vacuum actuator, the three solenoid valves are required to be fitted into the casing of the actuator while maintaining necessary sealing requirements. Typically, because such a vacuum actuator is required to be installed in a very limited space in an engine room of an automobile and is therefore required to be highly compact, a considerable difficulty arises when assembling the actuator. For instance, because it is desirable to provide the casing of the actuator with a means or surfaces for supporting the solenoid valve assembly for the purpose of assuring the necessary mechanical stability of the system and eliminating any wasted space within the casing, when solenoid valves are installed into the casing of the actuator, the rugged corners of the solenoid valve assembly may scrape off chips from the casing which is typically made of synthetic resin and thse chips could cause a failure of the actuator by impairing the proper functioning of the solenoid valves.